1. Field of the Invention
This invention relates to radiant energy detectors in the field of radiometry and photometry, and to standard instruments, such as luminance/illuminance sources, for generating stable calibrated quantities of radiant energy to be used, e.g., in calibrating photometric instruments.
2. Description of the Prior Art
In developing a luminance/illuminance source to be used for field calibration of photometers with a high degree of accuracy, e.g., within 3% of the photometric standards maintained by the National Bureau of Standards at a high level of user confidence, it is necessary to face the problem that in the field, calibration is highly dependent upon the stability of standard lamps and there is no way to guarantee the stability of a given lamp standard to within 3%. In order to achieve the required degree of reliability, it is necessary either to use redundant calibration with a statistically large sample of lamps, or to check the output of the calibration source each time it is used with a photometric instrument whose accuracy is itself better than 3%.
Present photometric instruments using silicon photovoltaic detectors are able to extend the range of linearity between incident radiant energy and output current over that available with previous selenium detectors. However, the requirement of a photometric instrument to stably measure luminance/illuminance within 3% relative to NBS standards is not being met by current instruments. Those devices which are stable over time are not generally stable over temperature and those which claim electrical temperature compensation are not spectrally stable over temperature. Present state of the art instruments generally are accurate to only within 5% relative to NBS standards at 25.degree. C.
As a result, present photometric instruments do not provide satisfactory accuracy and stability, and therefore cannot satisfactorily serve to maintain the output accuracy of a luminance/illuminance source to be used for calibration of photometers.